Transcription factors regulating the expression of the epidermal growth factor receptor gene include the transcriptional repressor UCF. UCF is a phosphoprotein that is primarily localized in the nucleus. UCF is encoded by a 3 kilobase RNA but the GCF cDNA also hybridizes to RNA species of 4.5 and 1.2 kilobases. Screening of cDNA libraries with the DNA fragment corresponding to the DNA binding domain of UCF resulted in the isolation of four cDNAs with homologous sequence. The cDNAs have homology to the DNA binding domain and hybridize to a 4.5 kilobase RNA but for the most part the DNA sequences are dissimilar. The DNA sequence of the UCF cDNA was compared to genomic sequence and found to contain a l base insertion (T) at position 787. This insertion causes a frame-shift but is in frame with two other potential translation initiation codons (211 and 347). This would give a protein with a predicted size of 85 to 90 kilodaltons, which is in good agreement with the size of (3CF immunoprecipitated from cells in culture. We have expressed a protein in cells using a vaccinia virus system that initiates translation at nucleotide 347. This protein is immunoprecipitable with anti-UCF and has a size of approximately 97 kilodaltons. This product can be synthesized in vitro using a coupled transcription/translation system. Two independently isolated cDNA clones have been selected from an FEM-X (melanoma) cDNA library and are homologous to GCF. Both clones lack the 320 bp at the 5' end of GCF that would give the DNA binding domain.